Resistances may be measured by comparing them with known resistances. These known resistances are usually made of coils of resistance wire contained in a box like that shown in Fig. 1.
For all coils except those that are of very small resistance, a wire is used that has a high specific resistance, i.e., one in which the value of is high, - such as German silver. For all coils, however, it is best to use a wire that has a low temperature coefficient. An alloy called platinoid, which has a temperature coefficient less than half that of German silver, is frequently used. When the terminals of a battery are connected with the binding posts of the box, the amount of resistance introduced into the circuit is determined by the number of plugs that are taken out. If all the plugs are in the box, the resistance is practically zero. If the plug T' (Fig. 2) corresponding to the 1-ohm coil is pulled out, the current, in going from the piece A to the piece B, must go through that coil, so that the resistance is 1 ohm. The resistances are generally 0.1, 0.1, 0.2, 0.3, and 0.4 ohm; 1, 2, 3, and 4 ohms; 10, 20, 30, and 40 ohms; and 100, 200, 300, and 400 ohms. This makes it possible to introduce any number of ohms from 0.1 ohm to 1111.1 ohms.
The coils are wound double, as shown in Fig. 2, to keep the coil from being a magnet as soon as a current passes through it, and also to prevent self-induction, a phenomenon that will be discussed later.
If the resistance coils are to be used as standards, great care is taken to prevent accidental changes in their resistance. Such a coil shown in Fig. 3 is wound on a cylinder which is inclosed in a metal tube. This tube is pierced with holes and is immersed in oil to keep the temperature of the coil constant. A thermometer can be inserted through the hole in the top on removing the plug. The terminals T' of the coil are very heavy, are carefully insulated, and contact is made by the use of mercury cups.
The rheostat is a kind of resistance box used for regulating dynamo and motor currents. The common ironclad rheostat consists of resistance wire wound in such a way that it can be put into or taken out of a circuit G by moving a metallic arm over a set of contact points.
Fig. 4 shows the face surface of a cast-iron enameled rheostat, Fig. 5 shows the under side.
The resistance wire is entirely embedded in the enamel, while the contact points project through it. The position of the arm determines the amount of resistance in the circuit.
